1. Field
The disclosed concept pertains generally to transfer mechanisms and, more particularly, to automatic transfer switches for selectively feeding power from one of two input lines to a load.
2. Background Information
Alternate power sources are provided for any number of applications, which cannot withstand a lengthy interruption in electric power. Typically, power is provided from a primary source with back-up power provided by a secondary source. Often, the primary source is a utility power source and the secondary source is an auxiliary power source, such as an engine driven generator or a second utility power source. The transfers between the two power sources can be made automatically or manually.
Transfer switches are well known in the art. See, for example, U.S. Pat. Nos. 7,948,117; 7,336,003; 6,849,967; 6,801,109; 5,397,868; 5,210,685; 4,894,796; and 4,747,061. Transfer switches operate, for example, to transfer a power consuming load from a circuit with a normal power source to a circuit with an auxiliary power source. Applications for transfer switches include stand-by applications, among others, in which the auxiliary power source stands-by if the normal power source should fail. Facilities having a critical requirement for continuous electric power, such as hospitals, certain plant processes, computer installations, and the like, have a standby power source, often a diesel generator. A transfer switch controls electrical connection of the utility lines and the diesel generator to the facility load buses. In many installations, the transfer switch automatically starts the standby generator and connects it to the load bus upon loss of utility power, and reconnects the utility power source to the load bus if utility power is reestablished.
Transfer switches commonly used to connect alternate power sources to a load, including networks, utilize a pair of power contacts each connecting one of the sources to the load. In order to prevent connecting unsynchronized sources together, the operation of the two power contacts is coordinated, typically by an interlock mechanism (e.g., mechanical and/or electrical), in order that only one power contact at a time can be turned on. In many instances, it is desirable to operate the transfer switch remotely. Typically, electric motors or solenoids have been used to operate the interlock mechanism on transfer switches. See, for example, U.S. Pat. Nos. 5,081,367; 4,760,278; and 4,398,097.
A transfer switch typically comprises a pair of power contacts, power contactors or circuit interrupters combined with a drive input and a linkage system. The preferred types of circuit interrupters have been molded-case switches and molded-case circuit breakers because these types are commercially available in a wide array of sizes and are relatively economical compared to other options. The preferred type of drive input depends on the application for the transfer switch. Usually motors or solenoids are preferred, but at other times there is a clear preference for manually-operated mechanisms.
Residential type automatic transfer switches, for example, have built-in time delays that allow for several steps to occur in sequence in order to ensure the least amount of interruption to the end user. For instance, there is a time delay engine start that allows for the utility to operate automatic reclosures before the automatic transfer switch starts the engine-generator. This time delay protects against un-needed engine starts.
Another time delay is the time delay before transfer to the engine-generator after it is running. This is typically done to allow the engine-generator to properly warm up before applying a load to it. Upon loss of utility power, the automatic transfer switch starts and warms up the engine-generator prior to transfer of the load to the generator. This warm up time is fixed.
There is room for improvement in automatic transfer switches.